Rhodium carbonyl coordination

Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

Under mild conditions, hydroformylation of olefins with rhodium carbonyl complexes selectively produces aldehydes. A one-step synthesis of oxo alcohols is possible using monomeric or polymeric amines, such as dimethylbenzylamine or anion exchange resin analog to hydrogenate the aldehyde. The rate of aldehyde hydrogenation passes through a maximum as amine basicity and concentration increase. IR data of the reaction reveal that anionic rhodium carbonyl clusters, normally absent, are formed on addition of amine. Aldehyde hydrogenation is attributed to enhanced hydridic character of a Rh-H intermediate via amine coordination to rhodium. 

IR data supported the formation of rhodium carbonyl anions on addition of amine suggesting a mechanism in which these anions are in equilibrium with the active species, a hydrido rhodium carbonyl containing coordinated amine. Such a mechanism provides an explanation for the variation in catalyst activity with amine concentration and basicity and affords a method for predicting the effect of other amine ligands. 

In situ IR and NMR spectra showed that, indeed, the assembled diphosphine coordinates preferentially in the bis-equatorial, wide-bite-angle fashion to the penta-coordinated rhodium carbonyl hydride. Both homo- and heterocombinations of monophosphine ligands can be formed in situ by mixing the components several heterocombinations perform better as catalysts than the respective homocombinations. However, in contrast to the catalysts prepared by Takacs, the present complexes do not equilibrate to afford solely the heterocombination. 

With the catalysts derived from (S,S)-l,2-bis(diphenylphosphinomethyl)cyclobutane and [RhH(CO)(PPh3)3] or rhodium carbonyls, the a,3-unsaturated aldehydes, neral and geranial, are hydrogenated to (E)- and (S)-citronellal in 79% and 60% ee, respectively. Cyclic a,3-unsaturated ketones such as isophorone and 2-methyl-2-cyclohexenone have been hydrogenated using ruthenium hydrides coordinated with chiral diphosphines in up to 62% ee to give chiral ketones, though conversions are not satisfactory. ... 

Among the complexes which may function in this way are pentacyano-cobaltate ion, iron pentacarbonyl, the platinum-tin complex, and iridium and rhodium carbonyl phosphines. It has been suggested that with tristriphenylphosphine Rh(I) chloride, a dihydride is formed and that concerted addition of the two hydrogen atoms to the coordinated olefin occurs (16). There are few examples of the homogeneous reduction of other functional groups besides C=C, C=C, and C=C—C=C penta-cyanocobaltate incidentally is specific in reducing diolefins to monoolefins. 

Neutral and anionic organoimidoimido clusters of rhodium)I) with diolefin and carbonyl ligands are accessible and stable. The diolefin neutral compounds [Rh4(/i-N-/7-tolyl)2(diolefin)4] are a convenient entry into this chemistry for which the synthetic route is an apparently simple replacement reaction, but the reproducibility of the results requires a careful technique. These tetranuclear compounds contain a trimetallic triangular cluster face capped on both sides by two pnra-toly-limido ligands and a further rhodium fragment coordinated to a para-to y ring in a 7 -fashion, giving an overall valence electron count of 64e. 

In iV-confused porphyrins, this agostic coordination mode has been observed frequently. To date, monomeric complexes of this macrocycle with divalent manganese (35, 26), divalent iron (36, 29), lanthanides (57), rhodium carbonyl (55), and group 12 elements (34), have been structurally characterized that exhibit this peculiar type of metal binding. A generic structure for several of... 

Catalytic processes that take place in a uniform gas or liquid phase are classified as homogeneous catalysis. Homogeneous catalysts are generally well-defined chemical compounds or coordination complexes, which, together with the reactants, are molecularly dispersed in the reaction medium. Examples of homogeneous catalysts include mineral acids and transition metal compounds (e. g., rhodium carbonyl complexes in 0X0 synthesis). 

Shortly after the report of the reaction of a rhodium carbonyl complex with O2 to give coordinated CO2, the catalytic oxidation of CO to CO2 was reported in a similar system [193]. Kiji and Furukawa reported that both [RhCl(COXPPh3)2] and [RhCl(COXMe2SO)2] catalyzed the oxidation of CO in benzene, ethanol, or dimethylsulfoxide. However, the rate was slow and the catalytic efficiency was poor. Reactions catalyzed by [RhCl(COXMe2SO)2] in benzene gave approximately 11m moles of CO2 per m mole of metal complex. The authors describe the reaction as involving (1) coordination of molecular oxygen, (2) reaction with CO in the coordination sphere, and (3) displacement of the product by unreacted CO. 

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